>
>> This can be detrimental to the health of the 3-500Z if the exciter is
>> capable of 150-200 watts, and no cathode padding or no external ALC
>> cut-back is used.
How is this detrimental to the tube? Surely, the 3-500Z will be driven
into saturation, however, the tungsten di-carbide emissive layer is
atomically-bonded to the tungsten/thorium filament--i.e., it can not be
stripped off. Slowly evaporated, yes. .
...snip...
>ALC will not cure the problem, because ALC is traditionally too slow
>to limit leading edges of waveforms.
verily
>Also be sure the radio actually keys the PA before RF is applied, and
>removes RF before the keying signal is removed. I've seen radios
>output RF long after the PA is unkeyed!
Which ones?
...snip...
>> The only failures I have had on 160 meters were the door
>> knob padding capacitors. That was before I obtained a book from HEC
>> and found out how low the current limits really were on such devices.
>> ETO, in their infinite wisdom, failed to supply higher current padding
>> caps with their 77DX-to-SX conversion kit. The drift in the original
>> cap due to heating was horrific! No wonder..its current rating is only
>> 1.6 amps @ 1 mhz!
Agreed. Apparently, Dick Erhorn is entitled to ignore cap. manufacturer
RF current ratings. OTOH, Dick is one of our "recognized experts".
[p.72 in the 9/94 QST]
>Be sure you apply that current rating correctly. On lower
>frequencies current is determined by the I*X voltage drop
>across the component and the voltage drop across the component. On
>160, you are well within the rating of the component. The 1.6 amperes
>does NOT indicate the capacitor is close to HEAT failure from
>excessive current, al low frequencies the current rating indicates
>the point of arcing from VOLTAGE failure.
-----Does this statement check out?
The 1000pF 58 series cap. is rated at 5kV. The current rating is 1.4A
@ 1MHz. The reactance of 1000pF @ 1MHz = -j159.15 ohms. Emax=159*1.4A =
222.6V rms or 314V peak--which doesn't seem very close 5000V. .......
IMO, the 1MHz rating is an issue of heating from RF-current---Not an
issue of voltage breakdown.
>Drift problems are a different issue. Drift problems have come and
>gone with ceramic doorknobs. HEC's capacitors were originally quite
>stable, while ITT Jennings caps were very poor. When HEC bought out
>ITT, HEC's stability went to hell.
According to Jennings' specifications, some doorknob caps. have NP0, some
N750, and some X5U temperature drift characteristics. The 500pF and
1000pF are examples of X5U characteristic units. Obviously, the X5U
caps. are unsatisfactory for use in any tuned circuit. However, for
coupling applications X5U caps seem to perform okay, provided the DC
voltage and RF current ratings are observed. Drift does not come and go.
It is a given in all N750 and X5U rated capacitors.
...snip...
>I've yet to see anything "strange" happen, only things that initially
>are not intitially understood. The key is to understand what is
>really going on.
>
>So if I'm missing something here, I certainly want to understand what
>it is!!!
You appear to be missing AC Circuit Analysis, Mr. Rauch. ..... Deja vu?
>A network behaves the same way at one watt as at a
>million watts, at least until something fails.
With air cores, yes---but not with saturable core materials.
...snip...
>The voltage at the anode end of the network is determined by the
>anode voltage, conduction angle of the tube, and energy storage in
>the tank. As the analysis point moves along the network towards
>the low impedance load, voltage steadily decreases. Someplace before
>the end of the inductor is reached, voltage to ground reaches a
>minimum (that's because a pi is really two back to back L networks,
>with a low Z point out a bit from one end).
In any tank circuit, energy storage is a given. IMO, calculating the
peak AC anode V is hardly as esoteric as proffered. The peak change in
anode voltage is: the anode supply voltage minus the anode-to-grid
voltage at the peak in anode current. . For a 3-500Z, this voltage is
about 250v. Thus, with a 3000v supply, the peak AC anode voltage is
3000V minus 250V = 2750v. This occurs during the decreasing swing in
anode potential. During the increasing swing in anode potential (during
which anode current ceases in Class AB service) the anode voltage peak
would be about 3000V plus 2750V = 5750V.
...snip...
>Other than the slightly higher voltage across the loading
>capacitor, the pi-L is no more likely to produce high voltages
>anywhere in the tank system than the pi.
good point
...snip...
>On a 80 through ten meter tank, the designer might get by without
>shorting unused turns, but he likely won't get away with it on a
>160 through ten PA.
However, on most amplifiers the 10m-12m-15m-17m L is not coupled to the
20m-40m-80m L.
...snip...
>No matter what the cause of this particular problem, I'd hate to
>see people dismiss pi-L's as "arc prone" systems.
Amen. Doing so is technoblatherous.
>73 Tom
Rich---
R. L. Measures, 805-386-3734, AG6K
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